Inside-to-outside flow filter tube and method of using same

ABSTRACT

An improved inside-to-outside flow filter tube and method of manufacturing the tube, which filter tube comprises: a plurality of nonwoven fibers having interstices therebetween to define the porosity of the filter tube, the tube containing a binding agent at the junction of the fiber crossovers to provide a self-supporting structure of a defined wall thickness and filter porosity; and an open scrim reinforcing sheet material within the wall of the filter tube extending generally the length of the tube and at least one overlapping revolution about the tube diameter, the fibers of the filter tube bonded integrally through the open scrim material, thereby permitting the use of the filter tube in applications requiring inside-to-outside fluid flow without the necessity for an external peripheral support.

This is a continuation of application Ser. No. 679,569, filed Apr. 23,1976 (now abandoned).

BACKGROUND OF THE INVENTION

Most disposable cartridge filters are usually operated withoutside-to-inside flow direction of the liquids or gases to be filtered.The outside-to-inside flow permits a greater dirt-holding volume in thefilter housing than if inside-to-outside flow were used. In addition, itis easier to support the filter tube to resist high differentialpressures when the flow direction is outside-to-inside, because thesmaller-diameter mechanical support (usually a perforated tube or porousrigid tube) can usually be fabricated from commercially availablematerials, while a larger-diameter support for the outside of thecartridge usually requires custom fabrication.

There is another reason that internal support is usually preferred overexternal support, which applies particularly to disposable cylindricalfilter tubes; for example, with approximately 1/8-inch wall thickness,made from a nonwoven random network of glass fibers 0.1 to 10 microns indiameter and bonded at the junction of the fibers by a hardenedmaterial, such as a resin. Filter tubes of this general type aredescribed, for example, in U.S. Pat. No. 3,767,054, issued Oct. 23,1973, hereby incorporated by reference. Filter tubes of this type arecommonly manufactured by forming onto the external surface of acylinder, either by depositing fibers onto a porous cylinder wall byvacuum, or by rolling a sheet of fibers onto the wall of the cylinder.As a result, the inside diameter of the filter tube is almost exactlythe same as the outside diameter of the forming cylinder, and,therefore, the inside diameter of the filter tubes can be made uniformand generally reproducible from tube to tube in production. For example,a control range of ± 0.005 inches on the tube internal diameter, or evenless, is easily achieved. However, since the outside diameter of thefilter tube is not confined during the forming process, control of thediameter is much more difficult, and the variability in outside diameterfrom tube to tube is much greater, typically ± 0.030 inches or more.

The variability in external diameter of filter tubes of this type hasmade the design and fabrication of external supports much more difficultand expensive than internal supports. For example, a reusable internalsupport of perforated metal or plastic (as shown in U.S. Pat. No.3,767,054) is quite feasible, because the close control of internaldiameter of the filter tubes assures that each filter tube will fit thesupport perfectly. It will be appreciated that satisfactory performanceof the support requires that the internal diameter of the filter tube belarge enough to permit the tube to be fitted; e.g., slid, over thesupport easily, and yet the internal diameter of the filter tube mustnot be considerably larger than the support core or else the supportcore will not adequately prevent rupture of the filter tube.

With the wide variability of outside diameters of the filter tubes, ithas been impractical to design a reusable external support core. Asupport core which is large enough to fit over filter tubes at the largeend of the diameter range will be too loose to support filter tubes atthe small end of the diameter range. Even a disposable external supportcore is difficult and expensive, because the support core must be strongenough to prevent the filter tube from bursting, yet malleable enough toconform closely to the variable outside diameter dimensions of thefilter tube. The external support problem may be solved by forming orcasting the filter tube inside a rigid perforated or porous support,such as a screen, but this manufacturing procedure is inherently moreexpensive and difficult than forming the filter tube on the outside of acylinder or mandrel.

Despite the difficulties in providing adequate dirt-holding capacity orburst strength for the filter tubes, there are definite advantages tofiltering fluids in the inside-to-outside direction under certainconditions and for certain purposes. For example, when coalescing andremoving liquid droplets from air or other gases, the inside-out flowdirection is essential to permit drainage of the coalesced liquid andminimize the chance of reentrainment and carryover of coalesced liquidby the gas (see Hydraulics and Pneumatics, August 1974, "CoalescingFilters Produce Clean Air for Fluidic Systems" (herein incorporated byreference).

Another example in which inside-out flow is desirable is in coalescingand separating of two or more liquid phases. In this procedure, thefilter collects and coalesces droplets of the dispersed liquid phase andproduces two distinct liquid phases which may then be easily separated.However, the coalescing action takes place throughout the depth of thefilter tube element, and the clean separation of the two liquid phasesoccurs on the downstream surface of the filter tube. If the flowdirection is outside-to-inside, the acceleration of the liquid as itleaves the relatively small flow area inside the tube tends to mix thetwo liquid phases and redisperse the discontinuous phase. However, ifflow is inside-to-outside, the liquid leaves the filter surface atminimum velocity, and the two liquid phases can separate cleanly withinthe filter housing.

A further example in which inside-to-outside flow is desirable is when atwo-stage treatment of the fluid is desirable; for example, contactingthe liquid with a sorbent material, such as an adsorbent clay ordiatomaceous earth, followed by filtration. If flow is inside-to-outsidethrough the filter tube, the loose powder, granular, or fibrous materialwhich is used for the sorbent pretreatment, either as a filter aid orprecoat material, or both, may be conveniently preloaded into the insideof the filter tube, and if necessary, held in place with perforated endcaps. The single disposable filter tube cartridge then convenientlyserves both to pretreat and filter the fluid. If flow wereoutside-to-inside through the filter tube, it would be difficult orimpossible to retain a precoat of powder, granular, or fibrous materialon the outside of the filter tube, and therefore a less convenient two-or multiple-step filter process would be required, such as is describedin Bulletin TI-62, 1973, of Balston, Inc. (hereby incorporated byreference).

SUMMARY OF THE INVENTION

Our invention relates to an improved filter tube and to the process ofmanufacturing and using such filter tube. In particular, our inventionconcerns an inside-to-outside flow filter tube of improved burststrength and of improved contaminant-holding capacity, and to theprocess of manufacturing and using such tube.

Our invention provides an improved disposable filter cartridge designedfor flow in the inside-to-outside direction. The improvement comprises,both alone and in combination: an improved tube and method ofconstruction which greatly increases the resistance of the filter tubeto bursting when the flow of fluid is in the inside-to-outsidedirection; and an improved tube and method of construction which greatlyincreases the contaiminant-holding capacity of the filter tube when flowof fluid is in the inside-to-outside direction.

In general, our objectives are achieved by incorporating an opensupporting scrim sheet material within the diameter of the tube; e.g.,the fiber structure of the tube, when the filter tube is being formed,and by packing the interior of the filter tube with a relatively largequantity of a coarser fibrous or particulate material which serves as apretreatment material; e.g., a precoat and/or prefilter for the fluid tobe filtered by the filter tube. It is understood that either one ofthese improvements may be used independently; that is, the improvedburst strength tube may be used without the internal pretreatmentmaterial, or the internal pretreatment material may be used with a tubeof standard burst strength and prior-art construction. Preferably, bothimprovements are used together, if the filtration situation requiresinside-out flow through the filter tube with improved life(contaminant-holding capacity) and improved burst strength.

Our invention comprises an inside-to-outside flow filter tube, whichfilter tube comprises: a plurality of nonwoven fibers having intersticestherebetween to define the porosity of the filter tube, the tubecontaining a binding agent at the junction of the fiber crossovers toprovide a self-supporting structure of a defined wall thickness andfilter porosity; and an open scrim reinforcing sheet material within thewall of the filter tube extending generally the length of the tube andat least one overlapping revolution about the tube diameter, the fibersof the filter tube bonded integrally through the open scrim material,thereby permitting the use of the filter tube in applications requiringinside-to-outside fluid flow without the necessity for an externalperipheral support.

In one embodiment, the scrim material is wrapped in an overlapping,contacting, generally circular relationship within the filter tube wall,particularly where the filter tube is manufactured by depositing thefiber slurry onto the external wall of a vacuum mandrel.

In another embodiment, the scrim material is wrapped in an overlapping,generally helically coiled, noncontacting relationship within the filtertube wall, with a layer of fibers between the scrim coil, particularlywhere the filter tube is manufactured by depositing the fiber slurryonto a flat screen and subsequently rolling on a mandrel.

In the former, the scrim material may comprise one or more distinct andseparate circular structures within the wall of the filter tube;although generally and preferably one scrim layer within the inner 50%of the wall depth may be used. In the latter embodiment, the helicalcoil extends helically through a major part, or less as desired, of thefilter tube wall, and extends from all, a part of or none of theinternal wall surface of the filter tube, depending on the size andposition of the scrim material used on the filter mat to a position suchthat the other end of the scrim material does not protrude or form aportion of the external wall of the filter tube.

Our process comprises the preparation of filter tubes composed of aplurality of random, nonwoven, inorganic fibers having intersticestherebetween to define the porosity of the filter tube, the tubecontaining a binding agent at the junction of the fiber crossovers toprovide a self-supporting structure of defined wall thickness andporosity, the improvement which comprises: positioning, during themanufacture of the filter tube, an open scrim reinforcing sheet materialwithin the wall thickness of the bonded filter tube, the sheet materialextending generally the length of the tube and at least one overlappingrevolution about the tube to provide an internal filter tube of a highburst strength.

In one embodiment, the fibers form a fiber slurry and are deposited in adesired amount and thickness onto the external wall surface of acylindrical porous mandrel, and the scrim material wrapped about thedeposited fibers and additional fibers from a fiber slurry are thendeposited over the wrapped scrim material, the additional fibers beingof the desired amount and thickness. If desired, a number of scrimwrappings may be used with additional fibers deposited between eachwrapping to provide enhanced burst strength; however, care must be takennot to employ in a thin wall thickness too many scrim layers or scrimwrappings so as to affect the structural integrity of the filter tube,such as to affect segregation of the tube into separate layers whichwould be detrimental to filter tube strength and performance. In themandrel coating technique, the scrim material is not desired on theexternal surface, while scrim, as a part or all of the internal surfacein the mat technique, is permissible.

The scrim material useful may comprise a wide variety of open poroussheet materials of sufficient flexibility to be rolled onto the filtertube wall and of sufficient strength to increase the burst strength ofthe filter tube. Typically, the openings which may be any shape, butpreferably are rectangular or square, may range from greater than about1/8 of an inch per opening which permits the fibers of the filter tubeto become entangled physically with the scrim material duringmanufacture of the tube, and such opening may range from 1/8; e.g., 1/4to 1 inch; e.g., 1/2 inch. The scrim material may be composed ofinorganic or organic woven or nonwoven fibers and representsubstantially a maximum open surface area to avoid interference withfluid flow or to alter the characteristics of the tube or its use. Thescrim, for example, may be of the same fiber as employed in the filtertube, and be inert, such as alumina, zirconia or glass fibers ormixtures thereof. Preferably, the same material is used for the scrim asin the fibers of the filter tube, so that there will be no change intube properties by a different material. Scrim material may be formed ofcarbon fibers, metal screen material or nylon, polyester or otherorganic fibers; although such organic fibers are not suggested forhigh-temperature use of the filter tube.

The filter tube wall thickness may vary as desired, but in typicalfilter tubes, the wall thickness would range from about 0.100 to 0.200inches; e.g., 0.125 to 0.150, with a fiber density of about 0.15 to 0.25grams/cc. Pretreatment materials used internally with our tubes, such asprefilter fibers, would be of much lower density, such as 0.05 grams/ccor less.

The scrim material should be overlapped at least one revolution, such asnot less than about 11/2 times, and preferably, up to 3 or 5revolutions. Too many revolutions should be avoided, particularly in themandrel method, to avoid a segregation of the wall thickness, while thenumber of revolutions with the mat technique varies with the length ofthe mat and scrim material and fiber thickness.

Pretreatment materials useful internally with the filter tubes of ourinvention encompass a wide variety of fibrous and particulate material(powder or granular) which may function for a variety of purposes, suchas prefiltering as with glass fibers, as sorbing materials to removeimpurities or other materials by adsorption or absorption, such as clayand diatomaceous earth materials, to remove color bodies from a gas or aliquid-like oil, as sterilizing agents like the use of metal salts, suchas silver salts on support materials like silica gel to sterilize waterand to filter out visible impurities, as reacting materials, such ascatalysts on a support to cause or effect a desired reaction, such as infilter tubes with a reinforced inorganic binder containing an internalnoble metal catalyst like platinum or an inert support-like silica, andwherein a high-temperature gas reaction is effected by simply passinghot gas from the inside to the outside of the filter tube, and asion-exchange agents by employing one or more layers of a suitableion-exchange resin in the interior of the tube as a pretreatmentmaterial to obtain a desired ion exchange. The pretreatment materials,when in fibrous form, may be packed within an internal perforatedcylinder within the internal diameter of the tube, or wrapped, such asin kit form, in a helical coil within the internal diameter with orwithout an internal porous support. Typically, end caps or other meansare used to retain the pretreatment materials in place, particularlywhere the material is in loose particulate form. The pretreatmentmaterial may, like the outside coalescing sleeve, comprise an open-cellfoam material inserted within the filter tube and adjacent and snuglyfitting the internal wall surface of the filter tube.

Optionally, a coalescing sleeve material is about and adjacent theexternal wall surface of the filter tube, with a typical materialcomprising an open-cell porous foam or fibrous material with a usualmaterial comprising an open-cell porous urethane foam sleeve to serve asan oil-coalescing material; although other porous materials may be used.

The fibers of the filter tube are nonwoven, randomly disposed fibersoften deposited from an aqueous slurry of the fibers, formed into tubeform, dried, and then impregnated with a suitable binding agent, anddried or cured to form a semirigid self-supporting filter tube which mayor may not be used with a porous support core, depending upon its use.Examples of fibers are alumina, zirconia and glass, particularlyborosilicate glass fibers. The fibers may range in diameter and for theapplications described are usually less than 10 microns; e.g., 0.001 to10 microns, such as 0.03 to 8 microns; e.g., 0.1 to 3.5 microns. Thebinding agents may vary, and include, but are not limited to, hardenedresins such as thermosetting or curable resins like phenol-formaldehydeand epoxy resins, as well as silicone resins, the oxides of the fibersused such as silica for glass fibers, and other materials used asbinding agents like quaternary ammonium silicates and the like.Preferably, the filter tubes are composed of glass fibers with hardenedresin binders; although inorganic binders are useful where the tube isemployed in high-temperature use.

In connection with the measurement of the strength of filter tubes, twogeneral strength measurement properties are employed. One property isrelated to the collapse strength of the tube which is that pressure(psi) at which, when evenly exerted onto the outside surface of thetube, causes the tube to collapse inwardly. This measurement is ofparticular importance where the tube is to be employed inoutside-to-inside fluid flow direction. The collapse strength of a tubedetermines under particular use conditions whether or not a poroussupport core is required.

Another property is related to the burst strength of the tube which isthat pressure at which, when evenly exerted onto the inside surface ofthe tube, causes the tube to burst outwardly. This measurement is ofparticular importance where the tube is to be employed ininside-to-outside fluid flow direction. The burst strength of the tubedetermines the maximum differential pressure rating to be placed on thefilter tube element, and determines whether or not an external supportis required in use. Both properties are of importance under someconditions, such as where the filter tube is used as a vent filter withflow occurring in both directions. However, in coalescing-type filters,where inside-to-outside flow is used or necessary, then the burst, andnot the collapse, strength of the tube is of significance.

Our filter tubes of improved burst strength and contaminant-holdingcapacity are particularly useful as filters for exhaust gases, such asfor exhaust gases of internal combustion engines, such as crankcasediesel or gasoline-powered automobile engines, for compressed airfiltration for air to instruments where an oil-free compressed air isdesired, for use in filtering vacuum pump exhaust gases which often havea visible oil mist from the oil of the oil seal of the vacuum pump, andfor separating a multiphase liquid, such as removing oil droplets in anaqueous stream, and for other applications where improved burst strengthand/or improved contaminant-holding capacity is necessary or desired.

Our invention will be described for the purpose of illustration only inconnection with glass-fiber filter tubes in particular applications.However, as is recognized by those persons skilled in the art, variouschanges and modifications may be made to these illustrative examples,which changes and modifications are and would be within the spirit andscope of our invention. The illustrative examples set forth are to thosefilter tubes and situations wherein inside-to-outside flow is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective partial cutaway view of a inside-to-outsidefilter tube assembly of our invention.

FIG. 2 is an enlarged cross-sectional view of the filter tube assemblyof FIG. 1 along the lines 2--2.

FIG. 3 is an enlarged cross-sectional view of another embodiment of afilter tube of our invention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an improved inside-to-outside flow filter tube assembly 10which comprises a filter tube wall 12 of bonded glass fibers and anopen, rollable, porous, scrim, woven sheet material 14 composed ofmultiple strands of glass fibers with generally square openings of about1/4 of an inch per side wrapped about three revolutions and positionedwithin the fiber wall 12 of the tube. A perforated metal tube 18 isdisposed internally within the filter tube with a wrapping of aboutthree to six times of a coarser glass-fiber prefilter mat 16 about themetal tube 18 between the internal wall of the bonded glass fibers 12and the outside surface of the tube 18 as a prefilter material.Optionally, the mat 16 is rolled within an open screen material. Therolled prefilter mat 16 is retained in place by end caps 20 and 22.About the external wall 12 of the tube is fitted an open-cell,urethane-foam-coalescing filter 24. The end cap 20 contains a centralopening for the positioning of a tie rod (not shown) or end flange (notshown) to position the filter tube. As illustrated, the metal tube 18extends at the one end slightly beyond the end cap 22 and the prefiltermaterial so as to fit into a fluid-flow opening, such as the outlet of adiesel exhaust crankcase vent opening. Axial compression of the ends ofthe filter tube effects a compression of the semirigid wall of fibers ateach end to form an end seal without the need for other gaskets. Fluidflow enters the inside of the filter and flows from the inside to theoutside successively through the prefilter glass fibers 16, the bondedglass-fiber filter tube wall 12, and the oil-coalescing filter sleeve24, as illustrated by the representative flow arrows of the drawing.

FIG. 2 is an enlarged cross-sectional view of the filter tube assemblyof FIG. 1, showing in particular the overlapping contacting of the scrimmaterial 14 within the filter tube wall 12.

FIG. 3 is an enlarged cross-sectional view of a filter tube prepared asset forth in Example 2; i.e., by the mat rather than the mandrel methodof Example 1. This drawing illustrates the helical coil of a scrimmaterial 32 in a filter tube 30, with the intervening wall of bondedglass fibers 34 of the tube separating the scrim material. In theillustration shown, most of the internal wall surface of the tube 30 issurrounded by the scrim material, with the other end of the scrimmaterial embedded in the bonded fiber wall of the tube, and notextending to the external wall surface of the tube. With arepresentative 2-inch internal diameter filter tube with a mat of 24inches in length and a scrim material of 8 to 20 inches, the revolutionsof the scrim would be about 11/2 to 3 revolutions. In the drawingillustrated, the scrim material is placed on the fiber mat about 1/2 ofan inch to 1 inch from one end prior to being rolled into tube form,dried and bonded.

EXAMPLE 1

A filter tube with improved burst strength is required to coalesce andremove liquid droplets; e.g., oil, from compressed air. The life ofpresent filter tubes is generally adequate for normal use, since the airis usually contaminated primarily with liquids which continuously drainfrom the filters, rather than with solids which tend to plug the filtertubes. However, surges in air flow, such as when a compressor starts up,make improved burst strength a most desirable commercial property.Approximately 5 to 20 grams (dry weight); e.g., about 10 to 12 grams, ofglass fibers, having a diameter of from about 1 to 2 microns, are formedfrom a water slurry of the fibers onto a vacuum mandrel 2-inch cylinderoutside diameter and 22 inches long. A glass-fiber woven scrim 20 inches× 211/2 inches, with approximately 3/8 of an inch square openings ofmultifilament glass fibers, is wrapped around the fibers on the formingcylinder so that the scrim forms approximately 3 contacting overlappingrevolutions around the formed fibers, virtually the entire length of thetube. An additional 40 to 60; e.g., 45 to 50, grams of fibers are thendeposited on the cylinder, so that the scrim is encapsulated within thecontinuous wall of the fibers of the filter tube so formed, andpositioned within the inner third of the internal wall thickness. Thetube is then dried and impregnated and bonded with epoxy resin as abinder. The reinforced tube had an average burst strength of 38.8 psiand an average collapse strength of 25.0 psi, while a nonreinforced tubemade by the same technique had an average burst strength of 25.0 psi andan average collapse strength of 19.8 psi.

EXAMPLE 2

A wet mat of borosilicate glass fibers formed from an aqueous dispersionof the fibers about 10 inches × 22 inches, containing 20 to 40 grams;e.g., 30 grams, of glass fibers (dry weight), is formed on a screen. Aglass scrim 91/2 inches × 211/2 inches, containing approximately 1/4 ofan inch square openings, is placed on the fiber mat, and then the matcontaining the scrim is carefully rolled into a cylinder on a vacuummandrel of 2-inch outside diameter. The filter tube so formed has theconstruction of generally three overlapping, but noncontacting, layersof scrim material in a generally helical coil form, with a layer ofglass fibers of selected depth separating each layer of scrim, the glassfibers forming a continuous wall through the openings in the scrim. Thewidth of the scrim material is selected based on the filter tube size,so that no scrim material is on or extends to the external filter tubewall, but rather ends short of the external wall surface. The tube isbonded with organosilicone resin binder as described in U.S. patentapplication Ser. No. 523,587, filed Nov. 14, 1974, herein incorporatedby reference (now U.S. Pat. No. 3,972,694, issued Aug. 3, 1976). Theaverage burst strength of tubes with such scrim reinforcing was 52 psi,while the burst strength of a tube of the same construction, onlywithout the helical coil of the scrim, was only 25 psi.

EXAMPLE 3

In other examples, other filter tubes were made by the methods describedabove, using different bonding resin contents, different compositionsand a different number of wraps of reinforcing scrim. In all examples incomparison with similar tubes, identical with the exception of the useof the reinforcing scrim, the data illustrate a significant increase inburst strength imparted by the scrim. Particularly in connection withthe helical coil tubes of Example 2, the increase is unexpected, sincethe scrim is simply helically wrapped within the tube. The collapsestrength data given show that our reinforcing technique is useful toimprove burst strength and has essentially no beneficial significanteffect on collapse strength, and, therefore, the reinforcing techniqueis particularly beneficial only and useful when the filter tube is to beused in the inside-to-outside flow direction.

EXAMPLE 4

It has been discovered further, and most unexpectedly, that the positionand placement of the scrim material, as set forth in Example 1, are ofimportance as to the burst strength of the tube. In the preferredembodiment of an overlapping contacting scrim material as in Example 1,the scrim material should be positioned less than a majority of the wallthickness from the internal wall surface, and typically less than about35%; e.g., 10 to 35%. The following test data illustrates the burststrength is increased as the scrim moves near the inside diameter.

    ______________________________________                                        Comparison of Filter Tubes                                                    Using Differing Scrim Positions                                               Average Burst Pressure (psi)                                                  ______________________________________                                        B-type filter tube                                                            (without scrim)  19.5     (range 17-21 psi)                                   B-type filter tube                                                            scrim near I.D.                                                               (1/3 of distance from                                                                          43.5     (range 40-48 psi)                                   I.D.)                                                                         B-type filter tube                                                            scrim near O.D.                                                               (2/3 of distance from                                                                          32.0     (range 25-38 psi)                                   I.D.)                                                                         ______________________________________                                    

EXAMPLE 5

A filter with increased capacity to retain dirt and viscous oildroplets, while maintaining very low flow resistance, is required as afume filter on diesel engine crankcase vent. Flow direction through thefilter tube is inside-to-outside, because a portion of the collected oilis sufficiently fluid to drain from the outside of the filter tube afterbeing coalesced. However, the viscous nondraining portion of the oilmist coats the fibers of a conventional filter tube and causes anunacceptable pressure drop in a short period of time. A 2-inch ID × 9inches long × 1/8 of an inch thick filter tube composed of borosilicateglass fibers is utilized to filter diesel engine crankcase exhaust, forexample, at a flow rate of about 1.5 CFM and 0.4 - 0.5 inches waterinitial pressure drop. At the end of 100 hours, the pressure drop acrossthe filter tube would rise to over about 3.0 inches water, anunacceptable level.

Next, an internal packing consisting of relatively low-density coarseglass fibers in random-batt form is rolled about a perforated metalinternal mandrel of 1-inch OD and is inserted into the center of afilter tube of the same size and composition as described in Examples 1and 2, and a means to retain the packing, such as end caps, insertedinto each end of the filter tube to retain the internal packing. Dieselengine crankcase exhaust is passed through the filter in theinside-to-outside direction. The improved filter operates significantlyover 100 hours before the pressure drop rises to an unacceptable level.The internal coarse glass-fiber packing traps most of the dirt andviscous liquid droplets, thereby preventing these materials from coatingthe relatively finer and denser glass fibers of the borosilicate fiberfilter tube. The tube is then able to function satisfactorily over arelatively long time period as a continuous coalescing filter for theless viscous oil droplets.

EXAMPLE 6

It is desired to separate a small quantity of oil droplets dispersed ina relatively dirty aqueous liquid. A filter tube composed ofborosilicate glass fibers with epoxy resin binder, 2-inch ID × 9 incheslong × 1/8 of an inch thick with a 1/8 of an inch thick external sleevecomposed of open-cell foam; e.g., urethan, is used to separate thesolution with flow in the inside-to-outside direction. The advantages ofinside-to-outside flow direction for this type of application have beendescribed above. The use of an open-cell urethane foam sleeve, oranother type of coarse fibrous or foam porous material, such as moldedcellulose or felted organic fibers, as a final separating element in acoalescing application, is well known and is not alone a part of ourinvention.

The filter tube described above performs an extremely efficientseparation of the two phases, reducing the oil content in the aqueousphase to less than 10 parts per million (ppm). However, its practicalapplications are quite limited, because the high dirt content in theliquid tends to block the flow passages of the filter rapidly, raisingthe pressure drop across the filter, and the relatively low burstpressure of the filter causes the filter element to rupture after a fewhours service. A filter tube with internal glass scrim reinforcing, madeas described in Example 2 above, is filled with about 20 to 40 grams ofrelatively coarse insulation-grade glass fibers rolled on a perforatedmetal mandrel, and a polyurethane open-cell foam sleeve was applied andfitted to the outside of the tube. When used to separate the oil-watermixture described above in this example, this filter tube gave the sameseparation efficiency as the standard tube, but, however, it provided a3-to-5-fold increase in time or useful life before exhibiting the samepressure drop as the standard filter tube. This improved filter tube wasable to function satisfactorily for an additional period of time,because the increased burst pressure permitted safe operation at higherdifferential pressure than the unsupported tube.

EXAMPLE 7

A borosilicate glass-fiber filter tube with a cured silicone resinbinder, reinforced with a glass scrim as described in Example 2, ispacked with a sorbent material; e.g., diatomaceous earth, around aporous internal flow distribution tube, and end caps are inserted in thetube to prevent the diatomaceous earth from being dislodged from theannular space between the porous inner flow distributor and the outerfilter tube. Water containing a slimy organic material, such as algae,is flowed through the filter from inside-to-outside and is filteredclean and bright with relatively good filter life. A standard filtertube without diatomaceous earth internal prefiltration is pluggedvirtually immediately by the slime. Inside-out filtration with thediatomaceous earth preloaded into the filter tube is far more convenientthan the conventional outside-in filtration with external precoat.However, inside-out filtration is practical only with theinternally-reinforced filter tube.

What we claim is:
 1. An inside-to-outside flow filter tube of improvedburst strength, which filter tube comprises:(a) a plurality of randomlydisposed, nonwoven glass fibers having a diameter of from about 0.001 to10 microns, and having interstices therebetween, the fibers bondedtogether into a self-supporting filter tube by a bonding agent at thejunction of the fiber crossover points to form a filter tube wall, and(b) an open scrim sheet material embedded within said filter tube wall,with no scrim material extending to the external filter tube wall, thescrim material extending generally the length of the filter tube and atleast about one and one-half revolutions about the tube diameter, thescrim material formed into a generally noncontacting helical scroll witha layer of glass fibers of selected depth separating each convolutedlayer of scrim material, the glass fibers bonded cooperatively with andthrough the scrim material by the bonding agent, to form a continuous,integrally bonded filter tube wall means of the glass fibers, saidfilter tube having an average burst strength of greater than about 52P.S.I.
 2. The filter tube of claim 1 wherein the glass fibers areborosilicate glass fibers.
 3. The filter tube of claim 1 wherein thediameter of the glass fibers ranges from about 0.03 to 8 microns.
 4. Thefilter tube of claim 1 wherein the ends of the filter tube, on axialcompression, form a self-sealing gasket against a surface at each end.5. The filter tube of claim 1 wherein the bonding agent is a hardenedresin material.
 6. The filter tube of claim 5 wherein the bonding agentis a thermosetting phenol-formaldehyde resin, an epoxy resin or asilicone resin.
 7. The filter tube of claim 5 wherein the bonding agentis silica or quaternary ammonium silicate.
 8. The filter tube of claim 1wherein the filter tube wall thickness ranges from about 0.1 to 0.2inches and the tube has a fiber density of about 0.15 to 0.25 grams/cc.9. The filter tube of claim 1 wherein the scrim material extends up toabout five revolutions about the tube diameter.
 10. The filter tube ofclaim 9 wherein the scrim material extends from about three to fiverevolutions about the tube diameter.
 11. The filter tube of claim 1wherein the scrim material extends generally uniformly throughout thefilter tube wall thickness.
 12. The filter tube of claim 1 wherein thescrim material extends concentrically about and is integrally bonded tothe internal wall surface of the filter tube.
 13. The filter tube ofclaim 1 wherein the scrim material has regular and generally uniformopenings of from about 1/4 to 1 inch in size.
 14. The filter tube ofclaim 1 wherein the scrim material comprises a glass-fiber scrimmaterial.
 15. The filter tube of claim 1 which includes a peripherallayer adjacent the internal wall surface of the filter tube of apretreatment material for the fluid to be filtered by the filter tube.16. The filter tube of claim 15 wherein the pretreatment materialcomprises a layer of glass fibers.
 17. The filter tube of claim 15 whichincludes an internal, perforated, tubular material centrally positionedwithin the filter tube, the pretreatment material retained between theinternal surface of the filter tube and the external surface of themandrel.
 18. The filter tube of claim 17 which includes a helicallywould mat of coarse glass fibers wound about the mandrel as thepretreatment prefilter material, the helical mat and mandrel snuglyfitted within the interior of the filter tube.
 19. The filter tube ofclaim 18 which includes cap means at each end of the filter tube betweenthe internal surface of the filter tube and the external surface of themandrel to seal and retain the pretreatment material.
 20. The filtertube of claim 1 wherein the filter tube includes an outer peripheralsleeve, about the external wall surface of the filter tube, of porousmaterial as a coalescing filter.
 21. An inside-to-outside flow filtertube of improved burst strength, which filter tube comprises:(a) aplurality of randomly disposed, nonwoven glass fibers having a diameterof from about 0.01 to 8 microns, and having interstices therebetween,the fibers bonded together into a self-supporting filter tube by ahardened resin bonding agent at the junction of the fiber crossoverpoints to form a filter tube wall; and (b) an open scrim sheet materialof glass fibers having openings of about 1/4 to 1 inch and embeddedwithin said filter tube wall, with no scrim material extending to theexternal filter tube wall, the scrim material extending generally thelength of the filter tube and from about one and one-half to fiverevolutions about the tube diameter, the scrim material formed into agenerally noncontaining helical scroll, with a layer of glass fibers ofselected depth separating each convoluted layer of scrim material, theglass fibers bonded cooperatively with and through the scrim material bythe bonding agent, to form a continuous, integrally bonded filter tubewall of the glass fibers, said filter tube having an average burststrength of greater than about 52 P.S.I.
 22. A method of filtering afluid stream, which method comprises:(a) introducing the fluid stream tobe filtered into the interior of an inside-to-outside flow filter tube,which filter tube comprises:(i) a plurality of randomly disposed,nonwoven glass fibers having a diameter of from about 0.001 to 10microns, and having interstices therebetween, the fibers bonded togetherinto a self-supporting filter tube by a bonding agent at the junction ofthe fiber crossover points to form a filter tube wall, and (ii) an openscrim sheet material embedded within said filter tube wall, with noscrim material extending to the external filter tube wall, the scrimmaterial extending generally the length of the filter tube and at leastabout one and one-half revolutions about the tube diameter, the scrimmaterial formed into a generally noncontacting helical scroll, with alayer of glass fibers of selected depth separating each convoluted layerof scrim material, the glass fibers bonded cooperatively with andthrough the scrim material by the bonding agent, to form a continuous,integrally bonded, filter tube wall means of the glass fibers; and (b)filtering the fluid stream by passage through the porous filter tubewall of the filter tube, said filter tube having an average burststrength of greater than 52 P.S.I.
 23. The method of claim 22 whereinthe fluid stream is a compressed air stream containing oil vapor to beremoved by the filter.
 24. The method of claim 22 wherein the fluidstream is an exhaust gas stream of an engine.
 25. The method of claim 22wherein the fluid stream is a liquid stream containing an immiscible,dispersed, liquid phase therein to be removed by the filter.
 26. Themethod of claim 22 which includes:(a) flowing the fluid to be filteredthrough a layer of coarse glass fibers forming a layer on the inside ofthe filter tube; and (b) after passing through the filter tube wallthrough a layer of porous coalescing material, surrounding the externalsurface of the filter tube.
 27. The method of claim 22 wherein thefilter tube includes prefilter material dispersed in the interior of thefilter tube, and which method includes passing the fluid stream from theinside to the outside through the prefilter material prior to filtrationby the filter tube wall.
 28. A method of filtering a fluid stream, whichmethod comprises:(a) introducing the fluid stream to be filtered intothe interior of an inside-to-outside flow filter tube, which filter tubecomprises:(i) a plurality of randomly disposed, nonwoven glass fibershaving a diameter of from about 0.01 to 8 microns, and havinginterstices therebetween, the fibers bonded together into aself-supporting filter tube by a hardened resin-bonding agent at thejunction of the fiber crossover points to form a filter tube wall, and(ii) an open scrim sheet material of glass fibers having openings ofabout 1/4 to 1 inch and embedded within said filter tube wall, with noscrim material extending to the external filter tube wall, the scrimmaterial extending generally the length of the filter tube and fromabout one and one-half to five revolutions about the tube diameter, thescrim material formed into a generally noncontacting helical scroll,with a layer of glass fibers of selected depth separating eachconvoluted layer of scrim material, the glass fibers bondedcooperatively with and through the scrim material by the bonding agent,to form a continuous, integrally bonded, filter tube wall of the glassfibers; and (b) filtering the fluid stream by passage through the porousfilter tube wall of the filter tube, said filter tube having an averageburst strength of greater than 52 P.S.I.